package DataStructures.Queues;

import java.util.ArrayList;

/**
 * This class implements a GenericArrayListQueue.
 *
 * <p>A GenericArrayListQueue data structure functions the same as any specific-typed queue. The
 * GenericArrayListQueue holds elemets of types to-be-specified at runtime. The elements that are
 * added first are the first to be removed (FIFO) New elements are added to the back/rear of the
 * queue.
 */
public class GenericArrayListQueue<T> {
  /** The generic ArrayList for the queue T is the generic element */
  ArrayList<T> _queue = new ArrayList<T>();

  /**
   * Checks if the queue has elements (not empty)
   *
   * @return True if the queue has elements. False otherwise.
   */
  private boolean hasElements() {
    return !_queue.isEmpty();
  }

  /**
   * Checks what's at the front of the queue
   *
   * @return If queue is not empty, element at the front of the queue. Otherwise, null
   */
  public T peek() {
    T result = null;
    if (this.hasElements()) {
      result = _queue.get(0);
    }
    return result;
  }

  /**
   * Inserts an element of type T to the queue.
   *
   * @param element of type T to be added
   * @return True if the element was added successfully
   */
  public boolean add(T element) {
    return _queue.add(element);
  }

  /**
   * Retrieve what's at the front of the queue
   *
   * @return If queue is not empty, element retrieved. Otherwise, null
   */
  public T poll() {
    T result = null;
    if (this.hasElements()) {
      result = _queue.remove(0);
    }
    return result;
  }

  /**
   * Main method
   *
   * @param args Command line arguments
   */
  public static void main(String[] args) {
    GenericArrayListQueue<Integer> queue = new GenericArrayListQueue<Integer>();
    System.out.println("Running...");
    assert queue.peek() == null;
    assert queue.poll() == null;
    assert queue.add(1) == true;
    assert queue.peek() == 1;
    assert queue.add(2) == true;
    assert queue.peek() == 1;
    assert queue.poll() == 1;
    assert queue.peek() == 2;
    assert queue.poll() == 2;
    assert queue.peek() == null;
    assert queue.poll() == null;
    System.out.println("Finished.");
  }
}
